There are several methods of measuring the oxygen concentration of liquids. For medical applications, electrochemical sensors have been developed and marketed. One instrument currently in use is the Rapidpoint 400, available from Siemens Healthcare Diagnostics, Inc. When measuring oxygen content of blood, a sensor of the type described in U.S. Pat. No. 5,387,329 is used. That sensor employs three electrodes, i.e. a working electrode, a reference electrode, and a counter electrode. The general principles of such three electrode sensors are described in U.S. Pat. No. 4,571,292. At the working electrode, oxygen is reduced to hydroxyl ions, while at the counter electrode the hydroxyl ions are oxidized to molecular oxygen. The sensors provide a reversible set of reactions and do not require consumption of the electrodes. The current measured when a voltage is applied across the working and counter/reference electrodes is correlated to the oxygen content of the sample.
Reference may be made to the description in U.S. Pat. No. 5,387,329 for details of a typical oxygen sensor. The three electrodes are thin metal strips deposited on a non-conductive substrate. In the '329 patent, the working electrode is positioned between the counter electrode and the reference electrode. An electrolyte layer, e.g. a Nafion® layer, which is activated when the sensor is in use, covers the electrodes. Next, the electrolyte layer is covered by a membrane that permits oxygen in a sample to diffuse through it to reach the working electrode. It is a feature of the '329 patent's sensor that the working electrode is very small and exhibits rapid non-depleting behavior.
The useful life of such sensors is of great importance, since typically they are available 24 hours a day in hospitals or other clinical settings. The '329 patent teaches that contamination of the membrane by sample components or by other impurities may affect the membrane, shortening its life. Delamination of the sensor components is also considered to be a cause of sensor inaccuracy or failure. Another problem relating to oxygen sensor life has been observed, which is overcome by the present invention.
Experience has shown that, rather than gradually losing accuracy, sensors may suddenly produce a spike in the current output that is unrelated to the oxygen content of the sample being tested, or to the oxygen content of wash or calibration solutions. Unless the current spike is only temporary, the sensor is useless and must be replaced. It is now believed that the current increase is caused by formation of small dendrites extending from the electrodes into the electrolyte layer, which cause an increase in current flow or cause a short circuit between the electrodes. Since the electrodes are quite small, as is the distance between them, it has been difficult to find a solution to this problem, while maintaining the present sensor size.
Contrary to a suggestion in the '329 patent, in a typical operation the sensor is normally polarized at the operating voltage, since it must be calibrated regularly and available for use at all times. After a sample has been tested for its oxygen content, the sensor compartment is washed using an aqueous wash reagent typically containing surfactant to remove the sample and the electrodes remain at the test polarization voltage (e.g. −0.800v) until needed, while remaining in contact with a segment of stagnant wash solution. The segment of wash reagent typically would contain a near ambient level of oxygen or would gradually equilibrate toward an ambient level of oxygen over time. Every 30 minutes, the sensor is tested with calibrating solutions to assure that the sensor is providing accurate results. Thus, the sensor is always kept active and exposed to oxygen in both the segment of stagnant wash and the calibrating solutions. Investigation has shown that this exposure to oxygen contributes to shortening the life of the sensors by the sudden appearance of current spikes as described above. The present invention relates to a means for increasing the life of oxygen sensors and avoiding the sudden appearance of current spikes, as will be described in detail below. The invention also may be applied to other amperometric sensors to improve performance and increase their service life.